Systems and methods for fabricating carbon nanotube-based vacuum electronic devices

ABSTRACT

Systems and methods in accordance with embodiments of the invention proficiently produce carbon nanotube-based vacuum electronic devices. In one embodiment a method of fabricating a carbon nanotube-based vacuum electronic device includes: growing carbon nanotubes onto a substrate to form a cathode; assembling a stack that includes the cathode, an anode, and a first layer that includes an alignment slot; disposing a microsphere partially into the alignment slot during the assembling of the stack such that the microsphere protrudes from the alignment slot and can thereby separate the first layer from an adjacent layer; and encasing the stack in a vacuum sealed container.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application claims priority to U.S. Provisional ApplicationNo. 61/728,955, filed Nov. 21, 2012, the disclosure of which isincorporated herein by reference.

STATEMENT OF FEDERAL FUNDING

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 U.S.C. 202) in which the Contractor has elected to retain title.

FIELD OF THE INVENTION

The present invention generally relates to the fabrication of carbonnanotube-based vacuum electronic devices.

BACKGROUND

Vacuum electronics is a broad term that typically references electronicdevices that rely on the application of the principles of electronemission in the context of a vacuum. For example, diodes, triodes,tetrodes, and pentodes, can be fabricated as vacuum electronic devices.Recently, carbon nanotubes (CNTs) have been studied for their potentialto provide for viable electron emitters within vacuum electronicdevices. Specifically, carbon nanotubes exhibit a host of propertiesthat would suggest that they could make for excellent field emitters,and can thereby enhance the functionality of vacuum electronic devices.Because of the numerous advantages that carbon nanotube-based vacuumelectronic devices can confer, there exists a need to improve theirmanufacture such that they can be made to be more commercially viable.

SUMMARY OF THE INVENTION

Systems and methods in accordance with embodiments of the inventionproficiently produce carbon nanotube-based vacuum electronic devices. Inone embodiment a method of fabricating a carbon nanotube-based vacuumelectronic device includes: growing carbon nanotubes onto a substrate toform a cathode; assembling a stack that includes the cathode, an anode,and a first layer that includes an alignment slot; disposing amicrosphere partially into the alignment slot during the assembling ofthe stack such that the microsphere protrudes from the alignment slotand can thereby separate the first layer from an adjacent layer; andencasing the stack in a vacuum sealed container.

In another embodiment, assembling the stack includes using automatic orsemi-automatic precision equipment to implement a pick and placeassembly technique to assemble the stack.

In yet another embodiment, the stack further includes a furtherelectrode.

In still another embodiment, the further electrode is one of: anextraction grid electrode, a gate electrode, and a focusing electrode.

In still yet another embodiment, the further electrode is one of anextraction grid electrode and a gate electrode, and the furtherelectrode includes one of: micromachined silicon grids and electroformedmetal mesh.

In a further embodiment, the further electrode includes electroformedmetal mesh that is one of: a TEM grid and a standard filter mesh.

In a yet further embodiment, the stack further includes a dielectriclayer.

In a still further embodiment, the dielectric layer is one of: aring-shaped mica and a ring-shaped ceramic.

In a still yet further embodiment, the thickness of the dielectric layeris between approximately 10 μm and approximately 100 μm.

In another embodiment, the first layer houses one of: the cathode, theanode, the further electrode, and the dielectric layer.

In yet another embodiment, assembling the stack further includesaffixing the spatial relationship of the layers and electrodes withinthe stack using one of: vacuum compatible epoxy, hard-mountingmechanical fixtures, and mixtures thereof.

In still another embodiment, encasing the stack in a vacuum sealedcontainer includes placing the stack in a standard vacuum tube package,evacuating the vacuum tube package to high vacuums, and hermeticallysealing the vacuum tube package.

In still yet another embodiment, encasing the stack in a vacuum sealedcontainer includes using a solder reflow bonding technique.

In a further embodiment, disposing a microsphere partially into thealignment slot includes using an end effector to place the microsphereinto the alignment slot.

In a yet further embodiment, the end effector is one of: vacuum tweezersand micromachined active grippers.

In a still further embodiment, assembling the stack further includestesting each layer of the stack for alignment accuracy.

In a still yet further embodiment, assembling the stack further includesattaching the cathode to an assembly platform using solder reflow orconductive epoxies that are vacuum compatible.

In another embodiment, the carbon nanotubes are grown onto a substrateincluding titanium.

In yet another embodiment, the grown carbon nanotubes are welded to thesubstrate.

In still another embodiment, the microsphere is disposed so as to helpalign the first layer with respect to an adjacent layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a process for fabricating a carbon nanotube-basedvacuum electronic device in accordance with embodiments of theinvention.

FIG. 2 depicts a holder that can be used to orient components within astack in accordance with embodiments of the invention.

FIG. 3 illustrates the layering of triode hybrid microassembly stack inaccordance with embodiments of the invention.

FIG. 4 illustrates the layering of a triode hybrid microassembly stackwithout the use of microspheres in accordance with embodiments of theinvention.

FIGS. 5A-5D illustrate a triode hybrid micro-assembly stack that can beassembled in accordance with embodiments of the invention.

FIG. 6 illustrates a DIP that can be implemented in accordance withembodiments of the invention.

FIG. 7 illustrates a schematic for the testing of an assembly stack thathas been assembled in accordance with embodiments of the invention.

FIGS. 8A-8F illustrate using a UV-Cured Epoxy to affix the spatialrelationship of the components within a diode hybrid microassembly inaccordance with embodiments of the invention.

FIGS. 9A-9F illustrate using thermal-cured epoxy to affix the spatialrelationship of the components within a diode hybrid microassembly inaccordance with embodiments of the invention.

DETAILED DESCRIPTION

Turning now to the drawings, systems and methods for proficientlyfabricating carbon nanotube-based vacuum electronic devices areillustrated. In many embodiments, fabricating a carbon nanotube-basedvacuum electronic device includes utilizing microspheres in assemblingthe stack of the constituent components to precisely align them prior tosealing them in a vacuum encasing. In a number of embodiments, layerswithin the stack include alignment slots that that accommodate themicrospheres, and thereby facilitate the aligning of the constituentcomponents. In several embodiments, the alignment of the componentlayers are repeatedly assessed during the assembly process to ensureaccuracy within tolerance. In this way, carbon nanotube-based vacuumelectronic devices can be fabricated with a high degree of precision.Moreover, many of the aspects of the fabrication processes are amenableto automation, or at least semi-automation. Accordingly, the fabricationprocesses can be used to produce bulk quantities of carbonnanotube-based vacuum electronic devices with a high degree ofprecision.

Microscale digital vacuum electronic devices have been studied for theirpotential application in extreme environments, e.g. where conventionalCMOS-based electronics may fail. For example, H. Manohara et al.disclose the manufacture of a microscale digital vacuum inverse majoritygate (H. Manohara et. al., Proc. of SPIE, Vol. 7594, pp. 75940Q-1 to75940Q-5 (2010)), that implements a fairly complex structure includingthree sets of field emitters, three distinct gate electrode structuresdividing the three sets of field emitters, and an overlaying anode thatis split into three corresponding sections. The anode is divided intothree sections so that the area of overlap can be reduced, and theMiller capacitance can be decreased. In this way, the inverse majoritygate can achieve high speed operation. H. Manohara et. al. propose thatsuch a device can be suitable for operation in extraterrestrialenvironments because of its ability to maintain operation under extremeconditions. Proc. Of SPIE, Vol, 7594, pp. 75940Q-1 to 75940Q-5 (2010),by H. Manohara et. al. is hereby incorporated by reference.

Notably, in many instances, carbon nanotubes (CNTs) are relied on as anelectron emission source in many microscale digital vacuum electronicsas they can provide for many advantages. For instance, CNTs are amongstthe strongest materials, as measured by tensile strength, and amongstthe stiffest materials, as measured by elastic modulus. Additionally,CNTs have also been determined to possess outstanding electrical fieldemission properties, with high emission currents at low electric fieldstrengths (e.g., applied field from 1-3 V/μm and an emission current˜0.1 mA from a single nanotube). Thus, CNTs are thereby attractive ascold-cathode field emission sources, especially for applicationsrequiring high current densities (hundreds to thousands of amperes percm2) and lightweight packages (high frequency vacuum tube sources).Indeed, in U.S. patent application Ser. No. 11/137,725 (issued as U.S.Pat. No. 7,834,530), Manohara et al. disclose particular configurationsfor a high density carbon nanotube-based field emitter that providefavorable performance characteristics. For example, Manohara et al.disclose that field emitters that include a plurality of bundles of CNTsdisposed on a substrate, where the diameter of the bundles is betweenapproximately 1 μm and 2 μm, and where the bundles of CNTs are spaced ata distance of approximately 5 μm from one another, demonstrateparticularly advantageous performance characteristics. U.S. Pat. No.7,834,530 is hereby incorporated by reference.

Unfortunately, the complexity of such structures can be disadvantageousfrom a fabrication perspective. Thus, H. Manohara et. al. proposedschemes for microscale digital vacuum electronics that are moreconducive to fabrication in U.S. patent application Ser. No. 13/796,943filed Mar. 12, 2013, the disclosure of which is hereby incorporated byreference. In particular, H. Manohara et. al. describe microscaledigital vacuum electronics that do not require the use of a gateelectrode. Still, in many instances, more complicated vacuum electronicdevices that do require the use of a gate electrode may be required;consequently, fabrication challenges may still exist in these cases.

Importantly, although the inclusion of CNTs within microsale digitalvacuum electronics can provide many advantages, the fabrication ofCNT-based vacuum electronic devices can be challenging. For example, inmany cases where CNT-based vacuum electronic devices that includemultiple electrodes were fabricated using monolithic integrationtechniques, the utilized processing techniques were generallysubstantially process intensive and resulted in low yield. This can beattributed to the difficulty of fabricating such intricate devices, andalso to the unique challenges that implementing CNTs pose. For example,it can be difficult to precisely govern the growth of CNTs, which isoften the last step of the fabrication processes that are typicallyused. Additionally, the CNTs are prone to movement (and thereby cancreate short circuits), and as a result, great care must be taken toensure the proper final alignment of the CNTs. Because of suchfabrication challenges, it has generally been difficult to fabricatesuch CNT-based microscale electronic devices in bulk quantities.

Thus, many embodiments of the invention provide for more proficientfabrication processes that can address some of the above-mentioneddeficiencies. For example, in many embodiments, the constituentcomponents are individually fabricated, subsequently stacked, andthereafter sealed in a vacuum encasing. Importantly, components withinthe stack can include features and/or subcomponents to facilitate theirprecise aligning. Thus, many embodiments include alignment slots,whereby aligning features, such as microspheres, can be used tofacilitate the aligning process.

Processes for fabricating carbon-nanotube based vacuum electronicdevices are now discussed in greater detail below.

Processes for Fabricating Carbon Nanotube-Based Vacuum ElectronicDevices

In many embodiments of the invention, the fabrication of CNT-basedvacuum electronic devices is made to be efficient and, in many cases,conducive to the bulk manufacture of the devices. In numerousembodiments, the constituent components are individually manufactured,and thereafter assembled into a stack that is to be encased in a vacuumsealed container; in a number of embodiments, the stack includesfeatures and/or subcomponents that facilitate alignment processes.

A process for fabricating CNT-based vacuum electronic devices isillustrated in FIG. 1. The process 100 includes growing 102 carbonnanotubes onto a substrate to form a cathode. Any suitable technique forgrowing the CNTs may be implemented, and the CNTs may be grown in anysuitable manner. For example, the CNTs may be grown in accordance withU.S. Pat. No. 7,834,530, incorporated by reference above; thus, they canbe grown as a plurality of nanotube bundles disposed on a substrate,where the diameter of the bundles is between approximately 1 μm andapproximately 2 μm, and where the CNT bundles are spaced at a distanceof approximately 5 μm from one another. As explained in U.S. Pat. No.7,834,530, such a configuration can yield favorable performancecharacteristics. Additionally, they can be grown in accordance with theguidance provided by U.S. patent application Ser. No. 14/081,932 to H.Manohara et. al., the disclosure of which is hereby incorporated byreference. U.S. patent application Ser. No. 14/081,932 discloses thatrobust carbon nanotube-based field emitters can be achieved bypatterning a substrate with a catalyst, growing carbon nanotubes on thecatalyst, and heating the substrate to an extent where it begins tosoften such that at least a portion of at least one carbon nanotubebecomes enveloped by the softened substrate. In this way, the carbonnanotubes can be well-adhered to the substrate. U.S. patent applicationSer. No. 14/081,932 discloses that the substrate may include titanium;accordingly, in many embodiments of the instant invention, the substrateincludes titanium. Of course, it should be understood that althoughseveral techniques for growing CNTs onto a substrate have beendiscussed, any suitable technique for doing so may be implemented inaccordance with embodiments of the invention.

The process 100 further includes assembling 104 a stack that includesthe cathode, an anode, and at least a first layer that includes analignment slot. Generally, the stack can define the body of theCNT-based vacuum electronic device. The stack can further include any ofa variety of constituent components in order to achieve the desiredfunctionality of the CNT-based vacuum electronic device. For example,the stack can include an additional electrode, such as an extractionelectrode, a gate/grid electrode, an accelerating electrode, and/or afocusing electrode; a dielectric layer; and/or layers that can house thecomponents within the stack. Generally, a gate/grid electrode can allowa relatively small variation in voltage to cause a significantly largevariation in the anode current. On the other hand, a focusing electrodecan be used to increase the electron beam density, thereby increasingthe current density, by focusing it onto an associated electrode that iswithin the stack. An extraction electrode can be used in associationwith a gate/grid electrode and can decrease the voltage required tomodulate the current (e.g. ˜<1V). By way of example, the extractionand/or gate electrodes can include one of: micromachined silicon grids(e.g. those developed at JPL) and electroformed metal mesh (e.g. TEMgrids or standard filter mesh, such as those sold by commercialentities, such as BUCKBEE-MEARS, INC.). Additionally, the dielectriclayer can be mica or other machinable ceramics. In some embodiments, thethickness of the dielectric layer is between approximately 10 μm andapproximately 100 μm.

Importantly, the constituent components can be individually fabricatedprior to assembly. In this way, each component may be customized so asto achieve desired characteristics. Thus, for example, the fabricationof each constituent component can implement fabrication techniquesirrespective of those used for other components. Additionally, theindividual components can be fabricated at a tailored level ofprecision. This level of flexibility can ultimately allow for thefabrication of more robust CNT-based vacuum electronic devices.Additionally, as the constituent components can be individuallyfabricated, they can be produced at a high level of fidelity (e.g.within a tightly specified tolerance). Where each constituent componentis produced in accordance with a high fidelity level, the resultingCNT-based vacuum electronic device can be of higher quality.

The first layer that includes an alignment slot can be associated withany component in accordance with embodiments of the invention. Forexample, in some embodiments, the first layer that includes an alignmentslot is a cathode holder. In a number of embodiments, the first layerthat includes an alignment slot is a gate holder. In severalembodiments, the first layer is the cathode substrate. In general, thefirst layer can be embodied by any component within the stack.

FIG. 2 depicts a holder that can be used to support components within astack in accordance with embodiments of the invention. The holder can beused to support for example a cathode or a gate in accordance withembodiments of the invention. In the illustration, the holder 200 isshown including a carve out 202 for accommodating a component within thestack, and further including alignment slots 204 that can be used tofacilitate the alignment of components within the stack. Of course itshould be understood that the alignment slots 204 can be of any suitableshape, and are not restricted to the form factor depicted in FIG. 2.

During the assembly of the stack, microspheres are disposed 106 withinthe alignment slots such that they protrude from the slot. In this way,they can help spatially orient components within the stack. FIG. 3illustrates how microspheres may be disposed within alignment slots tospatially orient components within a stack that is used to form a triodein accordance with embodiments of the invention. In particular, theillustration depicts the layering of a cathode 302, a cathode holder304, a gate 306, and a gate holder 308. As depicted in the illustration,microspheres 310 can be used to facilitate the proper alignment of thelayers. In particular, it is illustrated that the microspheres 310 aredisposed within the alignment slots 312, such that they protrude fromthem, and can thereby establish a desired separation distance betweenadjacent layers. In this way, microspheres can facilitate precisionalignment. The microspheres can be deposited in the alignment slotsusing any suitable technique. In some embodiments, end effectors areused to install the microspheres. In many embodiments, the end effectoris one of: vacuum tweezers and custom designed micromachined activegrippers. Of course, any suitable technique for placing the microspheresmay be implemented. It should also be noted that although microspheresare depicted and discussed, any suitable subcomponents can be used tofacilitate precision aligning between the layers in the stack.

In many embodiments, microspheres are not used to facilitate alignment,and instead, precision alignment is facilitated by components within thestack that include alignment slots. FIG. 4 depicts the layering ofconstituent components without utilizing microspheres in accordance withembodiments of the invention. In particular, a cathode substrate 402,and a gate 404, are aligned using plates with alignment slots 406.

In general, any compatible assembly techniques may be incorporated inaccordance with embodiments of the invention. For example, in manyembodiments, automation (and/or semi-automation) machinery can be usedto facilitate the assembly of the stack. Thus, in many embodiments, apick-and-place technique is used in conjunction with layering theconstituent components to form the stack in accordance with embodimentsof the invention. Automation and/or semi-automation can greatlyfacilitate the bulk manufacture of CNT-based vacuum electronic devices.In a number of embodiments the stack is affixed after assembly, andprior to final vacuum sealing. For example, in many embodiments, thestack is affixed using epoxy (e.g., UV-cured epoxy, or thermally curedepoxy). In a number of embodiments, the stack is affixed mechanically,e.g. using mechanical fasteners. In many embodiments, the constituentcomponents are layered and affixed intermittently. Thus, for example, insome embodiments, a first and second constituent component within astack are layered, and affixed to one another (e.g. via epoxy), prior tothe inclusion of additional constituent components; where additionalconstituent components are added, they can be affixed to the stack priorto the addition of further constituent components. In a number ofembodiments, the alignment of the stack is intermittently checked duringassembly to verify the efficacy of alignment processes.

FIGS. 5A-5D depict the assembled stack previously shown in FIG. 3 to beimplemented in the fabrication of a CNT-based triode vacuum electronicdevice in accordance with embodiments of the invention. In particular,FIG. 5A depicts a front view, FIG. 5B, depicts a top view, FIG. 5Cdepicts a side view, and FIG. 5D depicts a cross-sectional view of theassembled stack.

The assembly of the stack can be associated with any suitable baseunderlying substrate. For example, in many embodiments, the cathodesubstrate is affixed to an assembly platform using solder reflow orconductive epoxies that are vacuum compatible. In numerous embodiments,solder reflow bonding is used to affix the stack to conventionalpackages, such as leadless chip carrier (LCC) packages and dual in-linepackages (DIP). The bonding of a stack to a conventional package caninclude using solvent and/or O2 plasma to clean the package in a vacuumat 400° C. for 3 days, and attaching the stack to the package using 2mm² Au₈₀Sn₂₀ preform. FIG. 6 depicts a stack that is adhered to a DIP.More specifically, the stack 602 is depicted as being bonded to the DIP604.

After assembly, the stack may be tested to ensure viability prior tobeing encased in a vacuum sealed package. FIG. 7 depicts a testing rigthat can be used to test a stack that has been adhered to dual in-linepackages in accordance with embodiments of the invention. In particular,the testing rig 700 includes a stack 702, that itself includes a cathode704 and a grid electrode 706, that is bonded to a dual in-plane package,and further includes an anode 710. Wire bonded contacts 712 areelectrically coupled to the grid and the anode to allow for the testingof the stack.

The stack can be encased 108 in a vacuum sealed container. Any suitableencasing technique may be implemented. For example, in some embodiments,the stack is placed in a standard vacuum tube package, creating thevacuum (to a desired extent) within the package, and thereafterhermetically sealing the vacuum tube package. In many embodiments asolder reflow bonding technique is implemented in conjunction withencasing the stack in a vacuum-sealed container. In many embodiments, agetter is incorporated in the vacuum tube package to preserve thevacuum.

In some embodiments, the encasing is accomplished by: baking the stack(which may be bonded to a corresponding package) for 96 hours at 200° C.and 10⁻⁶ torr; firing the getter at less than 400° C., using IR shuttersto protect the vacuum tube encasing from heating; cooling the lid to300° C.; and soldering the lid at 300° C. and 10⁻⁶ torr. Although ofcourse, any suitable encasing technique may be used in accordance withembodiments of the invention.

The scope of the invention may be further understood with respect to thefollowing examples regarding proficient fabrication processes for thefabrication of CNT-based vacuum electronic devices in accordance withembodiments of the invention. In particular, FIGS. 8A-8F regardfabrication processes that implement UV-cured Epoxy to affix theconstituent components of a diode to one another to form a stack to beencased in a vacuum. More specifically, FIGS. 8A-8B depict thedispensing of UV-cured epoxy on a cathode. FIGS. 8C-8D depict thesubsequent deposition of a mica spacer on the epoxy-coated cathode.FIGS. 8E-8F depict the subsequent deposition of an anode, which isaffixed to the mica spacer using UV-cured epoxy.

Similarly, FIGS. 9A-9F depict the assembly of a stack to be used as thefoundation for a CNT-based diode vacuum electronic device, usingthermal-cured epoxy. In particular, FIG. 9A depicts the deposition ofMC7880 dots on the cathode; FIG. 9B depicts the deposition of the micaspacer on the cathode; FIG. 9C depicts the adhering of the mica spacerto the cathode; FIG. 9D depicts the deposition of MC7880 dots onto themica spacer; FIG. 9E depicts the adhering of the anode to the micaspacer; and FIG. 9F depicts curing the diode assembly on a hot plate.

Of course, it should be understood that the above described examples aremeant to be illustrative and not exhaustive. Any suitable techniques foraffixing the constituent components within a stack may be implemented inaccordance with embodiments of the invention may be implemented. Moregenerally, it should be understood that the above described systems andmethods are meant to be illustrative. In general, as can be inferredfrom the above discussion, the above-mentioned concepts can beimplemented in a variety of arrangements in accordance with embodimentsof the invention. For instance, the above-described processes are notrestricted to the fabrication of diodes and triodes; any suitableCNT-based vacuum electronic device can be fabricated in accordance withembodiments of the invention. For example, tetrodes and pentodes can befabricated. Accordingly, although the present invention has beendescribed in certain specific aspects, many additional modifications andvariations would be apparent to those skilled in the art. It istherefore to be understood that the present invention may be practicedotherwise than specifically described. Thus, embodiments of the presentinvention should be considered in all respects as illustrative and notrestrictive.

What is claimed is:
 1. A method of fabricating a carbon nanotube-basedvacuum electronic device, comprising: growing carbon nanotubes onto asubstrate to form a cathode; assembling a stack comprising: the cathode;an anode; and a first layer that includes an alignment slot; disposing amicrosphere partially into the alignment slot during the assembling ofthe stack such that the microsphere protrudes from the alignment slotand can thereby separate the first layer from an adjacent layer; andencasing the stack in a vacuum sealed container.
 2. The method of claim1, wherein assembling the stack comprises using automatic orsemi-automatic precision equipment to implement a pick and placeassembly technique to assemble the stack.
 3. The method of claim 2,wherein the stack further comprises a further electrode.
 4. The methodof claim 3, wherein the further electrode is one of: an extraction gridelectrode, a gate electrode, and a focusing electrode.
 5. The method ofclaim 4, wherein the further electrode is one of an extraction gridelectrode and a gate electrode, and wherein the further electrodecomprises one of: micromachined silicon grids and electroformed metalmesh.
 6. The method of claim 5, wherein the further electrode compriseselectroformed metal mesh that is one of: a TEM grid and a standardfilter mesh.
 7. The method of claim 3, wherein the stack furthercomprises a dielectric layer.
 8. The method of claim 7, wherein thedielectric layer is one of: a ring-shaped mica and a ring-shapedceramic.
 9. The method of claim 7, wherein the thickness of thedielectric layer is between approximately 10 μm and approximately 100μm.
 10. The method of claim 7, wherein the first layer houses one of:the cathode, the anode, the further electrode, and the dielectric layer.11. The method of claim 7, wherein assembling the stack furthercomprises affixing the spatial relationship of the layers and electrodeswithin the stack using one of: vacuum compatible epoxy, hard-mountingmechanical fixtures, and mixtures thereof.
 12. The method of claim 11,wherein encasing the stack in a vacuum sealed container comprisesplacing the stack in a standard vacuum tube package, evacuating thevacuum tube package to high vacuums, and hermetically sealing the vacuumtube package.
 13. The method of claim 11, wherein encasing the stack ina vacuum sealed container comprises using a solder reflow bondingtechnique.
 14. The method of claim 7, wherein disposing a microspherepartially into the alignment slot comprises using an end effector toplace the microsphere into the alignment slot.
 15. The method of claim13, wherein the end effector is one of: vacuum tweezers andmicromachined active grippers.
 16. The method of claim 7, whereinassembling the stack further comprises testing each layer of the stackfor alignment accuracy.
 17. The method of claim 7, wherein assemblingthe stack further comprises attaching the cathode to an assemblyplatform using solder reflow or conductive epoxies that are vacuumcompatible.
 18. The method of claim 7, wherein the carbon nanotubes aregrown onto a substrate comprising titanium.
 19. The method of claim 18,wherein the grown carbon nanotubes are welded to the substrate.
 20. Themethod of claim 7, wherein the microsphere is disposed so as to helpalign the first layer with respect to an adjacent layer.